Apple has released Security Update 2009-004 for Leopard and Tiger (PPC) via their Software Update utility and on the Web. According to the company, this update is recommended for all users and improves the security of Mac OS X. You can learn more about the security contents of this update over here. It should also be noted that previous security updates have been incorporated into this security update. The update requires Mac OS X 10.4.11 or later and weighs in at 10.7MB for Leopard, 166MB for Tiger, and 76.1MB for Tiger PPC.